Basalt has another talent we may be able to exploit

Olive Heffernan says coating farms with basalt could cool Earth by absorbing carbon dioxide (3 March, p 26). It could have another use: making hydrogen.

Basalt contains a similar amount of iron to peridotite, an igneous rock common in Earth's mantle but rare in the upper crust. In peridotite, a reaction named serpentinisation produces hydrogen as iron reacts with water under pressure. The upper crust contains lots of basalt, but mining it would not be necessary. I propose fracturing it to increase its surface area, and injecting alkaline water.